The present invention relates to axial air-gap electric machines, such as motors and generators, and, more particularly, to an electric motor or generator whose stator is adapted for high-power applications and low-voltage applications.
Electric drive systems have several advantages as propulsion systems for a wide variety of vehicles, ranging from light, one passenger vehicles, such as bicycles, to heavy tracked vehicles, such as military tanks. One is that an electric transmission system can be configured spatially with more flexibility than a mechanical system, leaving more room within the vehicle for its other systems. Another is that regenerative braking is easily implemented, using the motors as generators.
The Proceedings of the Thirteenth International Electric Vehicle Symposium (Japan, 1996) includes descriptions of many such vehicles. For example, Gary Starr and Rick Rocklewitz ("Electricycles for Zero Air Pollution & Fun") describe a 12 volt DC motor, having a total power of 400 watts and a peak current of 100 amps, for retrofitting to bicycles. At the other extreme of land vehicle size, an article entitled "Coilgun Research Spawns Mighty Motors and More", in the Sep. 24, 1993 issue of Machine Design, reports a 430 horsepower electric drive for military tanks, based on brushless DC motors.
One such tank motor, reported, by R. M. Ogorkiewicz, in an article entitled "Electric Tank Treads With More Torque, Less Noise", in Jane's International Defense Review 2/1966, consists of three or more units in tandem. Each unit has a rotor with permanent magnets around its circumference, rigidly attached to a common shaft, and rotating between two stators. The stators, mounted on the lateral walls of the units, included coils wound parallel to the walls on yokes attached to the walls and located at about the same radius from the shaft as the magnets. Space is at a premium in this application, and can be conserved in this motor by configuring it as a single unit instead of three separate units, by joining adjacent lateral walls and yokes to produce stators that consist of coils mounted on both sides of a central supporting member and that are sandwiched between pairs of rotors. This in and of itself is not new. Lee et al., in U.S. Pat. No. 5,146,144, disclose a motor operable in two different speed/torque regimes and including a stator comprising a central supporting member with coils on either side. Kaszman, in U.S. Pat. No. 4,761,590, teaches a reluctance motor having a stator sandwiched between two rotors and supported by special brackets. Further compression of the motor is possible, however, and is the subject of the present invention.